In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The background supporting such a rapid advance is a reduced wavelength of the light source for exposure. The change-over from i-line (365 nm) of a mercury lamp to shorter wavelength KrF excimer laser (248 nm) enabled mass-scale production of dynamic random access memories (DRAM) with an integration degree of 64 MB (processing feature size ≦0.25 μm). To establish the micropatterning technology necessary for the fabrication of DRAM with an integration degree of 256 MB and 1 GB or more, the lithography using ArF excimer laser (193 nm) is under active investigation. The ArF excimer laser lithography, combined with a high NA lens (NA≧0.9), is considered to comply with 65-nm node devices. For the fabrication of next 45-nm node devices, the F2 laser lithography of 157 nm wavelength became a candidate. However, because of many problems including a cost and a shortage of resist performance, the employment of F2 lithography was postponed. ArF immersion lithography was proposed as a substitute for the F2 lithography. Efforts have been made for the early introduction of ArF immersion lithography (see Proc. SPIE Vol. 4690, xxix, 2002).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water and ArF excimer laser is irradiated through the water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE Vol. 5040, p 724, 2003).
Several problems arise when a resist film is exposed in the presence of water. For example, the acid once generated from a photoacid generator and a basic compound added to the resist material can be partially leached in water. As a result, pattern profile changes and pattern collapse can occur. It is also pointed out that water droplets remaining on the resist film, though in a minute volume, can penetrate into the resist film to generate defects.
These drawbacks of the ArF immersion lithography may be overcome by providing a protective coating between the resist film and water to prevent resist components from being leached out and water from penetrating into the resist film (see 2nd Immersion Workshop: Resist and Cover Material Investigation for Immersion Lithography, 2003).
With respect to the protective coating on the photoresist film, a typical antireflective coating on resist (ARCOR) process is disclosed in JP-A 62-62520, JP-A 62-62521, and JP-A 60-38821. The ARCs are made of fluorinated compounds having a low refractive index, such as perfluoroalkyl polyethers and perfluoroalkyl amines. Since these fluorinated compounds are less compatible with organic substances, fluorocarbon solvents are used in coating and stripping of protective coatings, raising environmental and cost issues.
Other resist protective coating materials under investigation include water-soluble or alkali-soluble materials. See, for example, JP-A H06-273926, JP 2803549, and J. Photopolymer Sci. and Technol., Vol. 18, No. 5, p 615, 2005. Since the alkali-soluble resist protective coating material is strippable with an alkaline developer, it eliminates a need for an extra stripping unit and offers a great cost saving. From this standpoint, great efforts have been devoted to develop water-insoluble resist protective coating materials, for example, resins having alkali-soluble groups such as fluorinated alcohol, carboxyl or sulfo groups on side chains. See WO 2005/42453, WO 2005/69676, JP-A 2005-264131, JP-A 2006-133716, and JP-A 2006-91798.
As means for preventing resist components from being leached out and water from penetrating into the resist film without a need for a protective coating material, it is proposed in JP-A 2006-48029, JP-A 2006-309245, and JP-A 2007-187887 to add an alkali-soluble, hydrophobic compound as a surfactant to the resist material. This method achieves equivalent effects to the use of resist protective coating material because the hydrophobic compound is segregated at the resist surface during resist film formation. Additionally, this method is economically advantageous over the use of a resist protective film because steps of forming and stripping the protective film are unnecessary.
Independent of whether the resist protective coating material or alkali-soluble additive is used, the immersion lithography places the requirement of higher throughputs than ever from the standpoint of productivity. To attain the goal, the exposure time must be reduced. Exposure must be carried out by scanning the wafer-holding stage at a high speed of 300 to 700 nm/sec while water is partly held between the projection lens and the wafer. In the event of such high-speed scanning, unless the performance of the resist or protective film is sufficient, water cannot be held between the projection lens and the wafer. Water droplets are left on the surface of the resist film or protective film after scanning. It is believed that residual droplets cause defective pattern formation.
To eliminate the droplets remaining on the surface of the photoresist or protective film after scanning, it is necessary to improve the flow or mobility of water (hereinafter, water slip) on the relevant coating film and the water repellency thereof. It is reported that the defects associated with residual droplets can be obviated by increasing the receding contact angle of the photoresist or protective film with water. See 2nd International Symposium on Immersion Lithography, 12-15 Sep., 2005, Defectivity data taken with a full-field immersion exposure tool, Nakano et al. It is reported that introduction of fluorine into a base resin is effective for improving water repellency and formation of micro-domain structure by a combination of different water repellent groups is effective for improving water slip. See XXIV FATIPEC Congress Book, Vol. B, p 15 (1997) and Progress in Organic Coatings, 31, p 97 (1997).
One exemplary material known to have excellent water slip and water repellency on film surface is a copolymer of α-trifluoromethylacrylate and norbornene derivative (Proc. SPIE Vol. 4690, p 18, 2002). While this polymer was developed as the highly transparent resin for F2 (157 nm) lithography resist materials, it is characterized by a regular arrangement of molecules of (highly water repellent) α-trifluoromethylacrylate and norbornene derivative in a ratio of 2:1. When a water molecule interacts with methyl and trifluoromethyl groups, it orients via its oxygen and hydrogen atoms, respectively, and the orientation distance between water and methyl is longer. A resin having a regular arrangement of both substituent groups is improved in water slip because of a longer orientation distance of water. In fact, when this polymer is used as the base polymer in a protective coating for immersion lithography, water slip is drastically improved (see US 20070122736 or JP-A 2007-140446).
Another example of the highly water repellent/water slippery material is a fluorinated ring-closing polymerization polymer having hexafluoroalcohol groups on side chains. This polymer is further improved in water slip by protecting hydroxyl groups on side chains with acid labile groups, as reported in Proc. SPIE Vol. 6519, p 651905 (2007).
Although the introduction of fluorine into resins is effective for improving water repellency and water slip, the introduction of extra fluorine results in resins with a greater surface contact angle following alkaline development. As a result, those defects so called “blob defects” occur on the resist film surface (especially in the unexposed area) after development. It is believed that blob defects form during spin drying after development. The internal energy accumulating in residual water droplets on a resist film causes damages to the resist film surface in the spin drying step, which are observable as blob defects.
In general, the internal energy of a water droplet on a film is higher as the film becomes more water repellent. A fluoro-resin having high water repellency tends to induce blob defects. For the purpose of suppressing the occurrence of blob defects, the surface contact angle after development must be reduced in order to reduce the internal energy of a water droplet. One approach is to introduce highly hydrophilic groups (e.g., carboxyl or sulfo groups) into a resin, but these groups serve to reduce the water repellency and water slip of the resin, which becomes not applicable to high-speed scanning. There is a desire to have a resin material which can reduce a surface contact angle after development (so as to minimize blob defects) while maintaining highly water repellent and water slip properties during immersion lithography.
The highly water repellent/water slippery materials discussed above are expected to be applied not only to the ArF immersion lithography, but also to the resist material for mask blanks. Resist materials for mask blanks suffer from problems including a change of sensitivity during long-term exposure in vacuum and stability after coating. With respect to the control of sensitivity changes in vacuum, an improvement is made by a combination of acid labile groups of acetal and tertiary ester types (U.S. Pat. No. 6,869,744). It is believed that a resist film resulting from coating of a resist material varies its sensitivity or profile as an amine component is adsorbed to the resist film surface after coating. A method of modifying the surface of a resist film for preventing adsorption of an amine component to the resist film has been devised.